Design and realization of an apparatus for the continuous control of 41Ar release at the LENA chimney

We describe the realization of an apparatus for the continuous measurement of 41Ar released from LENA chimney. The apparatus is sensitive enough to determine activity concentrations as low as 102 Bq m-3. The detector is a 3' x 3' crystal NaI(Tl) scintillator with a 6 % energy resolution placed on the lateral surface of the superior part of the chimney. The pulses produced in the detector by the gamma-rays are amplified and conted by a 1024 channel analyzer. The total efficiency has been determined using a Monte Carlo program. The intrinsic efficiency of the detector has been calculated for a gamma-rays source uniformly distributed on its base surface; the geometric efficiency has been then calculated using a source uniformly distributed in the final cylindrical section of the chimney. The Monte Carlo program has been tested by comparing the intrinsic efficiency calculated for a point source versus energy with data reported in the literature. (orig.)